Metal sensors for finding metal or other magnetizable objects in walls, floors or in the earth use coils to excite magnetic fields and to measure changes of the magnetic flux. Objects are identified in that the measured flux changes differ depending on whether or not an object is present. The measured flux change is partly a direct consequence of the exciting magnetic field, and partly the consequence of a secondary magnetic field originating from the object. The secondary magnetic field originating from the object is generally a consequence of the exciting magnetic field.
Coil-based metal sensors are known which can estimate a distance of detected objects. Such devices operate in the prior art by the pulse induction method and estimate the distance on the basis of the decay behavior of the measured flux change. This estimation is dependent however on the material properties of the object and on the diameter of the object.
A metal sensor with two coils and a magnetic field sensor is known from the patent application, not yet published on the date of filing, having file reference DE 10 2010 043 078.1. Here, the coils are arranged in such a way that they generate a high magnetic field at the location of an object to be found, however the fields of the individual coils mutually compensate for one another at the location of the magnetic field sensor in the absence of a detectable object. If a detectable object is present, the secondary field originating from the object causes, at the location of the magnetic field sensor, a non-vanishing magnetic field, which can be detected by the magnetic field sensor. A favorable signal/noise ratio is thus produced. It is not possible however to estimate the distance of the object to be detected.